British Patent No. 2,023,339, Ainsworth, describes a lamp with a metal halidefill. This lamp has comparatively high output, for example 200 W, and is intended to be used as a projection lamp. The reflector diameter is comparatively small, having a cross section of about 6 cm. The fill of the lamp must be carefully selected, so that the light which is generated is, essentially, "white" light, that is, light having a high color temperature in the order of 5000 K, see also U.S. Pat. No. 4,396,292.
Color temperature generally is a function of output power and the overall temperature conditions of operation of the lamp. The impression of the spectral distribution of the light delivered by the lamp is obtained by superposition of various emission lines. The mercury emission lines and the emission lines of metal halides are superposed. The respective emission lines of light of the metal halides depend highly on the vapor density and pressure within the discharge vessel. For a high color temperature as used in projection lamps, a fill having low vapor density and pressure of the metal halides is sufficient as the emission of the metal halides is to a large extent in the long wave range of the visible spectrum, the main portion of the emission spectrum of the fill - metal halides and mercury - lying withing the shorter wave range of the visible spectrum.
Reflector lamps with metal halide fills suitable for general service illumination, that is, for example for ordinary room illumination, and particularly such lamps of low power, require a lower color temperature than projection lamps. The color temperature should be substantially lower with respect to projection lamps and, preferably, should be at about 3500 K. With respect to the fill, this corresponds to a high vapor density and pressure of the metal halides to obtain an increased proportion of long-wave radiation within the emission spectrum.
Lamps suitable for general service illumination, that is, for example for general interior room illumination, should have power ratings of between 35 to 70 W, that is, comparatively low power. On the other hand, the color temperature of lamps used for room illumination requires high vapor pressure and density of the metal halides, for example of sodium which, however, is difficult to obtain due to the over-proportional thermal losses which occur in operation of low power lamps. In reflector lamps for projection, even for small power, metal halide fills are quite suitable. However, unless special measures are used, the vapor pressure and density obtained therein is not sufficient for general room illumination.
Various solutions have been proposed to permit the use of metal halide fill reflector-type discharge lamps for general service illumination; one such solution is described in German Patent Disclosure Document DE-OS 28 40 771. This is a double-ended lamp, which is located axially within an external bulb or housing element formed as a reflector. The heat balance and heat distribution obtained is enhanced by an open cylindrical tube which surrounds the discharge vessel within the outer bulb element, and is held therein by a comparatively complex holding structure. Such a construction is difficult and expensive to make. The lamp is comparatively heavy and has a long axial dimension.